1. Field of the Invention
This invention relates to thermotropic (melt processable) liquid crystalline polymers (LCP's) and more specifically to aromatic polyesters containing Bisphenol A (BPA) in combination with 4,4'-thiodiphenol (TDP) to provide improved moldability, color, shrink resistance and resistance to thermal oxidation.
2. Brief Description of the Prior Art
Thermotropic liquid crystalline polymers are thermoplastic materials which can be melt processed to yield products of exceptionally high performance characteristics. Key advantages of these materials are:
Outstanding melt processability, even in thin sections and in intricate components. PA1 Extremely high strength and modulus, PA1 High impact strength coupled with high temperature capability, PA1 Excellent chemical resistance, even at high temperatures, PA1 Excellent dimensional stability, including a near zero coefficient of thermal expansion and low moisture adsorption. PA1 m is 0 to 0.75, PA1 n is 0.125 to 0.5, PA1 p is 0 to 0.375, PA1 q plus t is 0.125 to 0.25, and PA1 n equals p plus q plus t, PA1 m plus 2p is 0.5 to 0.75, PA1 t is greater than 0.0125, but equal to no greater than 90 % of q plus t. PA1 a) p-Hydroxybenzoic acid, PA1 b) a member selected from the group consisting of hydroquinone and p-aminophenol, PA1 c) Bisphenol A, PA1 d) 4,4'-thiodiphenol, and PA1 e) terephthalic acid,
A more detailed review of properties, applications and markets has recently been published by J. R. Dole, Chemtech., 17, 242 (1987).
Thermotropic LCP's are made predominantly with aromatic moieties containing para linkages. They are basically modifications of 1,4-phenylene polyesters or polyesteramides. The parent polymers, polyhydroxybenzoic acid (PHBA), poly(1,4-phenylene terephthalate) (P(HQ/TA)), and poly(1,4-phenylene terephthalic esteramide) (P(AP/TA)) are too high melting and crystalline to be processable. Their melting points (Tm) are in the range of 600.degree. C. A P(HBA/TA/HQ) (50/25/25) copolymer still has a Tm in 1980.The latter two approaches have an advantage over the first two approaches in that the linear nature of the polymer chain is retained which leads to preservation of liquid crystallinity and the development of better physical properties. However, the cost of the monomers used in these approaches to achieve the desired property modification, is generally high.
Commercial LCP's, sold under the trade marks Vectra, (Hoechst-Celanese) and Xydar, (Amoco Chemicals, Inc.) , use the third approach with comonomers based on 2,6-naphthalene, poly(p-oxybenzoate-co-2,6-oxynaphthoate), and p,p'-phenylene, poly(p,p'-biphenylene terephthalate co-p-oxybenzoate), respectively. More recent commercial entries, Dupont's HX series and Granmont's Granlar, use the fourth approach and are believed to be poly(p-phenylene-terephthalates) based on phenylhydroquinone and 1-phenethylhydroquinone respectively.
The structure property relationships in LCP'S containing the structural variations mentioned in Table I have recently been reviewed in the paper by W. J. Jackson, Jr. as well as by a number of others earlier (e.g., J. Jackson, Jr., Brit.Poly.J., 154-162, December 1980, D. R. Wilson and S. R. Jones, Symp. Proc., "Polymers in the Service of Man", Div. of Ind. & Eng. Chem., Am. Chem. Soc., 78-85, June 9-11, 1980, and G. W. Calundann and M. Jaffe, Proceedings of the Robert A. Welch Foundation, Conference on Chemical Research XXVI, Synthetic Polymers, Houston TX, 247-287 (1982)).
The use of BPA as a flexible monomer to reduce melting point has been well reported in the literature. Jackson, Jr. (Brit. Poly.J., 154-162, December 1980) reports that BPA is effective in reducing the polyester melting point but is particularly unsuitable in that liquid crystallinity is lost and physical properties much reduced in an HBA/TA/BPA (54/23/23) polyester. In general, bisphenols with one atom linking the aromatic rings are recognized as reducing liquid crystallinity.
Griffin and Cox have also noted the greater disruptiveness of BPA vs TDP in other liquid crystalline polymers. Additionally, J.-I. Lin, S. Antoun, C. Ober and R. W. Lenz, Brit.Poly.J., 132-146, December 1980, disclose the effect of BPA and other bisphenols in reducing the liquid crystallinity of polyesters based on chlorohydroquinone and TA. Liquid crystallinity was retained at up to 20 mole % BPA while with TDP up to 30 mole % could be tolerated. With a series of polymers based on methylhydroquinone and TA they showed that there was no liquid crystallinity at 25 mole % BPA. BPA also reduced Tm slightly more than TDP. Thus, up to about 25 mole % BPA can be tolerated in 1,4-phenylene LCP systems. However, when other monomers are also used to depress Tm less BPA can be tolerated.
As disclosed in U.S. Pat. No. 3,637,595, BPA has been used to improve the heat distortion temperature (HDT) of their liquid crystalline polymer based on HBA, HQ and IA. IA is used to reduce processing temperature of the polyphenylene polyester but results in a very low HDT. As shown in Table II Tg is significantly increased by levels of BPA below 5 mole % but that above this point melts which were anisotropic become isotropic.
TABLE II ______________________________________ COPOLYESTER OF HBA/IA/HQ/BPA Mole % Anisotropic HBA IA HQ BPA Tm .degree.C. Tg .degree.C. Melt ______________________________________ 33.3 33.3 33.3 -- 333 110 Yes 33.3 33.3 30.0 3.3 320 175 Yes 33.3 33.3 28.3 5.0 315 181 Yes 33.3 33.3 26.6 6.7 -- 190 No 50.0 25.0 25.0 -- 360 115 Yes 50.0 25.0 20.0 5.0 360 190 Yes ______________________________________
Physical properties for the first composition in the Table are tensile strength 17,800 psi, elongation 10 % and notched Izod of 2.5 which shows that very high physical properties are attained even when considerable kinking monomer (IA) is used, as long as the melt is still liquid crystalline.
Polyesteramides formerly derived from polymerization of an aminophenol (as opposed to a bisphenol) with a dicarboxylic acid are of particular interest because of the potential for improved strength and toughness due to interchain hydrogen bonding. While structurally similar polyamides are much higher melting than their polyester analogues, the polyesteramides melt in the same vicinity as the polyesters and possibly even lower. This is excess of 500.degree. C. Modification of these polymers by copolymerization with monomers having the structural units of the types shown in Table I has resulted in polymers with reduced Tm's which can be melt process.
TABLE I __________________________________________________________________________ MODIFYING STRUCTURAL UNITS FOR REDUCED MELTING TEMPERATURE STRUCTURE TYPE EXAMPLES __________________________________________________________________________ (1) Flexible Linkages, ##STR1## CH.sub.2 CH.sub.2 (2) Nonlinear Linkages, ##STR2## (3) Other Rigid Units With Linearly Opposed Linkages, ##STR3## ##STR4## (4) Linear Linkages With Asymmetric Substituents. ##STR5## __________________________________________________________________________
More importantly, depending on the type and amount of modification many of these copolymers retain order in the molten state, i.e., are thermotropic LCP, which leads to most of the property advantages mentioned above. The presence of alkylene or alkyleneoxy spacers between aromatic rings such as in the first approach, or halogen or alkyl substituents such as in the last approach, however, limits the thermal and oxidative stability of LCP's as disclosed in W. J. Jackson, Jr., Mol.Cryst. Liq.Cryst., 169, 23-49 (1989). Use of non-linear linkages as in the second approach are known to rapidly decrease liquid crystallinity as is also disclosed by W. J. Jackson, Jr., and B. P. Griffin and H. K. Cox, Brit.Poly.J., 154-162, December believed due in part to the head-to-head or head-to-tail arrangement that the aminophenol residues may assume relative to each other which would lower crystallinity. Thus, for example, the softening points and melt behavior of some polyesteramides derived from p-aminophenol and the isostructural polyesters from hydroquinone are compared in Table III.
TABLE III ______________________________________ P-AMINOPHENOL AND HYDROQUINONE DERIVED POLYMERS SOFTENING POINTS .degree.C. DICARBOXYLIC POLYESTER- POLY- MELT BE- ACID AMIDE ESTER HAVIOR ______________________________________ 2-Methoxyterephthalic 155 254 Anisotropic 2-Butoxyterephthalic 205 270 Anisotropic p-Carboxyphenoxyacetic 330 355 Anisotropic p-Biscarboxymethoxy- 210 210 Isotropic benzene ______________________________________
U.S. Pat. No. 3,398,212, to Jackson, Jr. et al, discloses the use of thiodiphenol and/or variously substituted thiodiphenols as a comonomer to impart oxidative stability to polycarbonates and polyesters derived from bisphenols and diacids where at least 10 mole % of the bisphenols contain a "saturated bicyclic atomic-bridged hydrocarbon ring member". The use of such bicyclic structures introduces secondary and tertiary saturated hydrocarbon groups into the polymer and increases the polymer softening and melting temperatures. These groups are much prone to oxidative degradation and typically require the use of antioxidants. The patent recognizes that while it has been proposed to use thiodiphenol in polymers, such polymers may have poor thermal properties, such as poor glass transition temperature. The use of equimolar amounts of TDP and BPA was noted as resulting in a polymer having a glass transition temperature of only 125.degree. C. and it is recognized that there is a need in the industry for polycarbonates and polyesters which have improved oxidative stability and good tensile and thermal properties. In The British Polymer Journal, December 1980, pages 154-162, patentee Jackson, Jr. discusses approaches to reducing the high melting points of rod-like polyesters. The effectiveness of modifiers in reducing the polyester melting points has been found. However, the tensile and flexural properties and liquid crystallinity was found to generally decrease as the amount of modifier was increased, with BPA being particularly bad in this respect.
U.S. Pat. No. 4,219,629 to Storm, discloses copolyesters of hydroxybenzoic acid with a thiodiphenol and an aromatic dicarboxylic acid. The purpose of including the thiodiphenol is to make the hydroxybenzoic acid polymer soluble and more flexible. Solubility was particularly desired so that coatings of substrates could be made. The patent in addition to 4,4'-thiodiphenol, discloses 3,3'-thiodiphenol and 3,4'-thiodiphenol as well as the monoesters and diesters of such compounds.
U.S. Pat. No. 4,477,647 to Mark discloses the use of thiodiphenols in polyarylate resins to achieve flame retardancy and improved impact strength. No mention is made of any improvements in oxidative stability (other than flame retardancy).
U.S. Pat. Nos. 4,678,825, 4,680,371, 4,803,236, and 4,829,113 to Rosenfeld relate to the stabilization of polyesters resulting from the polymerization of terephthalic/isophthalic acids with bishydroxyphenyl compounds. The preferred bishydroxyphenyl compound is Bisphenol A, and others are disclosed, including 4,4'-thiodiphenol, and combinations of bisphenols. The patents discuss the problem of oxidative stability with aromatic polyesters derived from bisphenols and dicarboxylic acids. The specific problems noted are of discoloration and depolymerization resulting from exposure to the high temperatures (ranging up to 400.degree. C.) encountered in melt polymerization and processing. The patent does not distinguish between the bishydroxyphenols which might be used and their relative needs for stabilization. Additives included in the patents for improved stability are: 2,5-(dimercapto)-1,3,4-thiadiazole, 2-mercaptobenzthiazole, triphenylphosphite, a diphosphite based on Bisphenol A, and a mixture of the latter phosphite and poly(phenylene oxide).
U.S. Pat. No. 4,075,173 to Haruyama is a process patent for the solid state molecular weight advancement of a prepolymer. The patent fails to recognize either the criticality of the composition of the instant invention or the benefit of including 4,4'-thiodiphenol with Bisphenol A to improve oxidative stability. The patent discloses ranges of hydroxybenzoic acid to Bisphenol A of 1:100 to 100:1 in combination with terephthalic and/or isophthalic acid. The disclosure further indicates that part of the diacetate of Bisphenol A may be replaced by a small amount of other bisphenols among which are listed hydroquinone, bis(4-hydroxyphenyl)sulfone and bis(4-hydroxyphenyl)thioether. Neither preferred compounds nor ratios of compounds is disclosed which address the problems noted by Jackson, Jr..